The invention relates to a quadrature coil system for use in a magnetic resonance apparatus. The quadrature coil system includes first and second electric main conductors which are situated at a distance from one another in a first plane and a second plane, respectively, and each of which is formed as a loop with an opening. The main conductors having mainly the same orientation. The quadrature coil system further includes first and second connection conductors, each of which constitutes an electric connection between free ends, situated to corresponding sides of the opening, of the first and second main conductors. In addition, electric connection means are included which are connected to the conductors so that there are formed first and second sets of turns which are suitable to generate and/or receive first and second magnetic fields, respectively, which are mutually orthogonally directed.
An example of such a quadrature coil system is known from U.S. Pat. No. 5,057,777 which describes a quadrature coil system which is particularly suitable for examination of, for example the wrist of a patient by means of MRI (magnetic resonance imaging). The open ring represented by the path of each of the main conductors is shaped therein as a U. In the known quadrature coil system the conductors are connected to the connection means at oppositely situated ends of the coil system. The electric currents produced upon excitation at one side of the coil system then generate a horizontally directed RF magnetic field and the currents produced upon excitation at the other side produce a vertically directed RE magnetic field. It is a drawback of the known coil system that it must be connected to a transmitter or receiver at two remote points. Consequently, equalization currents will flow between the two connection points because these connection points are interconnected via interconnections outside the coil system the transmitter or receiver. These equalization currents cause undesirable interference fields whereby the image homogeneity and the signal-to-noise ratio could be affected, and also cause undesirable losses in the equalization path, which losses may also be detrimental to the signal-to-noise ratio. A known method of reducing this drawback, briefly indicated in the cited document, is the insertion of so-called RF traps in the connections whereby RF surface currents are blocked. It is a drawback, however, that these RF traps must be comparatively large so as to be effective. Moreover, it is necessary to shield these traps in order to prevent coupling to the environment, which would again give rise to disturbances. These shields themselves can also produce undesirable eddy currents which could disturb the RF field as well as the gradient field, thus affecting the image. Moreover, the traps require a time-consuming adjustment.